A high-molecular weight polyolefin has been widely used as an industrial material since it has a high chemical stability, is excellent in mechanical properties and is inexpensive. On the other hand, the use of a low-molecular polyolefin is limited to waxes. However, a low-molecular polyolefin is expected to attain a higher level of functionality.
An attempt to improve the functionality of a polyolefin has been conducted for many years. However, in addition to the fact that no effective manufacturing technologies have been established for a low- to middle-molecular weight region, a technology of adding polarity or the like to a polyolefin as a hydrocarbon has been restricted. In particular, an attempt to product a low- to middle-molecular weight polyolefin by a metallocene catalyst has been made in recent years. However, this attempt has its limit for introducing a functional group such as an unsaturated group which is necessary to impart a higher degree of functionality.
In order to introduce an unsaturated group, heat decomposition of a high-molecular weight polyolefin, in particular, polypropylene, is disclosed (Patent Documents 1 and 2, and Non-Patent Document 1). Patent Document 1 discloses propylene (the number of vinylidene groups per molecule is 1.8, for example) obtained by decomposing isotactic polypropylene at 370° C. Patent Document 2 discloses a heat-decomposable polybutene (the number of vinylidene groups per molecule is 1.53 to 1.75, for example) obtained by heat decomposing polybutene at 370° C.
In Patent Document 1 and Patent Document 2, the number of vinylidene groups is increased by highly decomposing a high-molecular weight substance. However, this method has a problem that the yield is lowered due to the generation of a large amount of bi-products.
Non-Patent Document 1 discloses a heat decomposed polypropylene (the number of vinylidene groups per molecule is 1.66 to 1.80). It discloses that, in respect of the relationship between the molecular weight and the number of vinylidene groups, it is difficult to allow the number of vinylidene groups per molecule to be 1.8 or more, particularly 2.0 or more. Further, by the method disclosed in Non-Patent Document 1, polypropylene is obtained only in the form of a mixture of both terminally saturated polypropylene, one terminally saturated polypropylene and both terminally unsaturated polypropylene, resulting in a poor yield.
As mentioned above, it is difficult to control an unsaturated group generated at the decomposition terminal to attain a high degree of unsaturation, and hence, it was impossible to produce an intended product efficiently. In addition, the resulting polyolefin has a high tacticity [mmmm] or does not have tacticity.
In addition to the method by heat decomposition, a method for controlling a terminal unsaturated group by using a catalyst (a metallocene catalyst, for example) is disclosed (Patent Document Nos. 3 to 6).
In Patent Document 3, atactic polypropylene having terminal vinylidene groups is produced by polymerization by using a zirconocene dichloride catalyst. Patent Document 4 discloses an example of producing isotactic polypropylene having terminal vinylidene groups by using polypropylene having tacticity [mmmm] of 88.8 and 94%. Patent Document 5 discloses highly-pure and highly-selective polyolefin having a terminal vinylidene group which is obtained by using low tactic polyolefin having tacticity [mmmm] of 30 to 80%.
However, by this method, it is impossible to attain a high degree of unsaturation since the number of terminal unsaturated groups is at most 1.0 per molecule.
Patent Document 6 discloses a method of introducing an unsaturated group by polymerization. Specifically, a method of introducing a large amount of diolefin residues per molecule by an ethylene/propylene/diolefin copolymerization is disclosed.
However, in the case of copolymerization with diolefin, it is difficult to introduce an unsaturated group at the terminal, and hence, there is a problem that a side reaction such as cross linking or the like proceeds during the polymerization reaction.
Of the above-mentioned terminally-unsaturated polyolefin, polyolefin showing tacticity is mainly highly crystalline isotactic polypropylene. However, in order to allow it to be used in various applications, control of plasticity, hardness and conditions such as liquidity and solidity is required. Under such circumstances, a polyolefin base material of which the molecular weight is in the low to middle range, and has a large number of terminal unsaturated groups and controlled tacticity has been required.